The first galaxy with three supermassive black holes was discovered.

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Supermassive black holes act as engines that feed the centres of galaxies, including our galaxy, the Milky Way. But, for the first time in history, an international team of astronomers led by scientists from the universities of Göttingen and Potsdam, have discovered that a galaxy contains three supermassive black holes in its core and that they are surprisingly close together.

That’s right. The first known galaxy that has three supermassive black holes has been found.

The irregular galaxy, NGC 6240, which is relatively close, 300 million light-years away, has an unusual shape, which previously led scientists to believe that it formed when two smaller galaxies collided and began to fuse.

Because of its “proximity”, experts have been able to study it through light waves. It was seen as a standard for galactic interaction and scientists thought that because of fusion, there were probably two black holes at its core. However, they were surprised when they identified a third supermassive black hole at the center of the galaxy. Each of them has the mass of more than 90 million suns, according to observations made by the Very Large Telescope of the European Southern Observatory in Chile (ESO). 

This new finding suggests that not two, but three galaxies are in the process of merging, each of which contributes its own galactic nucleus of supermassive black hole.

"Through our observations with extremely high spatial resolution," explained astrophysicist Wolfram Kollatschny of the University of Göttingen in Germany, "We were able to demonstrate that the interactive galaxy system NGC 6240 houses not two, as previously supposed, but three supermassive black holes in its center".

By way of comparison, the supermassive black hole in the center of the Milky Way, Sagittarius A*, weighs about 4 million solar masses. In addition, the three giant black holes of NGC 6240 are located in a region of less than 3,000 light-years in diameter, which is less than 1% of the size of the galaxy in which they reside. They are incredibly close together.

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P Weilbacher (AIP), NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

"Until now, such a concentration of three supermassive black holes had never been discovered in the universe," said Peter Weilbacher, of the Leibniz Institute for Astrophysics Potsdam, co-author of the study. Although astronomers have previously found three separate galaxies and their associated black holes on a collision course before, this is the first time they have witnessed the presence of supermassive black holes crammed into such a small space.

The discovery of this triple system is of fundamental importance for understanding the evolution of galaxies over time. So far it has not been possible to explain how the larger and more massive galaxies we know of our cosmic environment in the "present time", were formed solely by the interaction of normal galaxies and fusion processes over the course of the previous 14 billion years; that is, the age of our universe. "However, if there were simultaneous fusion processes of several galaxies, then larger galaxies with their central supermassive black holes could evolve much faster," concludes Peter Weilbacher. "Our observations provide the first indication of this scenario".

The findings are not only strange, exciting and an unprecedented discovery, it also shows how several galaxies can join together to build the largest galaxies in the universe. However, it will take at least another billion years for this three-fold merger to take place. Will humanity still be here to see it?

The study was published in Astronomy & Astrophysics.

 Reference: W. Kollatschny et al, NGC 6240: A triple nucleus system in the advanced or final state of merging, Astronomy & Astrophysics (2019). DOI: 10.1051/0004-6361/201936540

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